The present invention relates to interferometers. It finds particular application in conjunction with maintaining alignment of interferometers and will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other applications.
Michelson interferometers are known and have been used in many commercial applications. Also well known is that slight misalignment of the optical elements cause modulation changes that can significantly affect the performance of the interferometer. There have been numerous attempts in the design of commercial interferometers, Michelson interferometers included, to assure that either misalignments have been reduced, or that effects of misalignments have been reduced. Some of these attempts include passive means such as using cube corner mirrors, retro-mirrors, and/or other means to compensate undesirable effects. Others have used active means such as dynamic mirror alignment or active thermal control, among others. Alternatively, readily accessible adjustment mechanisms are made available to the user or maintainer of the interferometer that allows for periodic or necessary reestablishment of the relationships of the optical components to maintain an acceptable alignment condition.
The functioning of a Michelson interferometer is well known, based on the design of Michelson in 1891. Griffiths and deHaseth begin their book “Fourier Transform Infrared Spectroscopy” describing the operation of the Michelson interferometer. It is necessary to have a movable mirror that maintains its perpendicularity and flatness to a wave front while the mirror is moving or has moved to a new position. Any short or long term change (commonly referred to as optical instability) in the perpendicularity or flatness of either the fixed or movable mirrors to the wave front may produce compromised results. Similar results occur if the beam splitter changes flatness or angle relative to the wave front. While the effects of mirror misalignment are described in the Griffiths and deHaseth book, the authors make little attempt to address the specifics of the underlying causes of optical instability or loss of modulation efficiency.
Historically, interferometers have been designed with significant mass and thermal capacity for the purposes of reducing the misalignment effects of mechanical, acoustic, and thermal disturbances. Obviously, instruments using massive interferometers are not easily portable, or even easily movable at best. More recently there have been instruments designed for portability and which are designed to maintain alignment. One such instrument, described by Korb, et al., (Applied Optics, 1 Apr. 1996) and patented by Dybwad (U.S. Pat. No. 5,173,744) is reported to have been used for 3 years without the need for realignment. Unfortunately, the instrument and interferometer described therein requires the use of infrared transmitting prisms that require very stringent manufacturing tolerances, resulting in significant manufacturing costs.
More recently Simon et al. (U.S. Pat. No. 5,309,217) invented an interferometer using a pivot and retro-reflectors. Although the Simon et al. patent states the interferometer is stable, easily aligned, and a compact configuration, the invention presented in that patent requires a plurality of additional mirror surfaces and an increase in associated path length that significantly contributes to optical instability resulting from thermal change. Furthermore, the added optical elements and associated structure certainly challenge the use of the word “compact.”
Flat spring/bearings have been used with interferometers. However, until now, such flat spring/bearings have required periodic realignment when significant temperature changes occur in the interferometer's operating environment. Although the means for realignment is typically achieved by means of an automatic alignment algorithm, actuated through precision stepper motors, the need for a realignment often occurs at inopportune times, which causes significant user inconvenience and/or frustration. In addition, significant costs (both monetary and space) are incurred to effect realignments.
The present invention provides a new and improved compact and portable interferometer system that maintains a condition of substantially permanent alignment without the need for expensive prisms or additional expensive optical elements.